Physiological functions of mineral macronutrients.

Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

†Background Productive agriculture needs a large amount of expensive nitrogenous fertilizers. Improving nitrogen use efficiency (NUE) of crop plants is thus of key importance. NUE definitions differ depending on whether plants are cultivated to produce biomass or grain yields. However, for most plant species, NUE mainly depends on how plants extract inorganic nitrogen from the soil, assimilate nitrate and ammonium, and recycle organic nitrogen. Efforts have been made to study the genetic basis as well as the biochemical and enzymatic mechanisms involved in nitrogen uptake, assimilation, and remobilization in crops and model plants. The detection of the limiting factors that could be manipulated to increase NUE is the major goal of such research. †Scope An overall examination of the physiological, metabolic, and genetic aspects of nitrogen uptake, assimilation and remobilization is presented in this review. The enzymes and regulatory processes manipulated to improve NUE components are presented. Results obtained from natural variation and quantitative trait loci studies are also discussed. †Conclusions This review presents the complexity of NUE and supports the idea that the integration of the numerous data coming from transcriptome studies, functional genomics, quantitative genetics, ecophysiology and soil science into explanatory models of whole-plant behaviour will be promising.

REVIEW: PART OF A SPECIAL ISSUE ON PLANT NUTRITION Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

Languishing for many years in the shadow of plant inorganic nitrogen (N) nutrition research, studies of organic N uptake have attracted increased attention during the last decade. The capacity of plants to acquire organic N, demonstrated in laboratory and field settings, has thereby been well established. Even so, the ecological significance of organic N uptake for plant N nutrition is still a matter of discussion. Several lines of evidence suggest that plants growing in various ecosystems may access organic N species. Many soils display amino acid concentrations similar to, or higher than, those of inorganic N, mainly as a result of rapid hydrolysis of soil proteins. Transporters mediating amino acid uptake have been identified both in mycorrhizal fungi and in plant roots. Studies of endogenous metabolism of absorbed amino acids suggest that L- but not D-enantiomers are efficiently utilized. Dual labelled amino acids supplied to soil have provided strong evidence for plant uptake of organic N in the field but have failed to provide information on the quantitative importance of this process. Thus, direct evidence that organic N contributes significantly to plant N nutrition is still lacking. Recent progress in our understanding of the mechanisms underlying plant organic N uptake may open new avenues for the exploration of this subject.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02751.x

Uptake of organic nitrogen by plants.

Demand of all living organisms on the same nutrients forms the basis for interspecific competition between plants and microorganisms in soils. This competition is especially strong in the rhizosphere. To evaluate competitive and mutualistic interactions between plants and microorganisms and to analyse ecological consequences of these interactions, we analysed 424 data pairs from 41 (15)N-labelling studies that investigated (15)N redistribution between roots and microorganisms. Calculated Michaelis-Menten kinetics based on K(m) (Michaelis constant) and V(max) (maximum uptake capacity) values from 77 studies on the uptake of nitrate, ammonia, and amino acids by roots and microorganisms clearly showed that, shortly after nitrogen (N) mobilization from soil organic matter and litter, microorganisms take up most N. Lower K(m) values of microorganisms suggest that they are especially efficient at low N concentrations, but can also acquire more N at higher N concentrations (V(max)) compared with roots. Because of the unidirectional flow of nutrients from soil to roots, plants are the winners for N acquisition in the long run. Therefore, despite strong competition between roots and microorganisms for N, a temporal niche differentiation reflecting their generation times leads to mutualistic relationships in the rhizosphere. This temporal niche differentiation is highly relevant ecologically because it: protects ecosystems from N losses by leaching during periods of slow or no root uptake; continuously provides roots with available N according to plant demand; and contributes to the evolutionary development of mutualistic interactions between roots and microorganisms.

/pdf/competition-between-roots-and-microorganisms-for-nitrogen-17bzfyqeu0.pdf

Competition between roots and microorganisms for nitrogen: mechanisms and ecological relevance

Effects of organic and inorganic nitrogen sources on growth and mineral nutrient concentrations of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were compared in a 100-day experiment in a greenhouse. Seedlings were grown in pots containing peat. Nutrient solutions differing in ammonium, nitrate, arginine and glycine composition were supplied to the seedlings at three nitrogen (N) concentrations: 1, 3 and 10 mM. We used dual (13C, 15N) and single (15N) isotopic labeling to determine the uptake of organic and inorganic N at the end of the experiment. Seedling dry weights and mineral nutrient concentrations of the needles showed that both conifer species were able to grow well and maintain nutrient balance on all investigated N forms except for the ammonium-dominated nutrient mixtures at the 10 mM N concentration. In Scots pine, no significant differences in dry weights were found between seedlings grown on the amino acids and seedlings grown on a commercial fertilizer containing 61.5% NO(3-)-N and 38.5% NH(4+)-N. Isotopic labeling of seedlings indicated that uptake rates of arginine-N, glycine-N and NH(4+)-N were similar, and 7-8 times greater than uptake rates for NO(3-)-N in both species. In Scots pine seedlings, 100% of arginine-N, and at least 67% of glycine-N was derived from the uptake of intact amino acids through seedling roots or mycorrhizae. Corresponding figures for Norway spruce were 83% for arginine and 96% for glycine. The gas chromatography-mass spectrometry analyses confirmed the presence of intact labeled molecules of both arginine and glycine in seedlings. We conclude that arginine and glycine are comparable to inorganic N as N sources for growth of conifer seedlings.

/pdf/growth-of-conifer-seedlings-on-organic-and-inorganic-5doribi16n.pdf

Growth of conifer seedlings on organic and inorganic nitrogen sources

Plants have the ability to take up organic nitrogen (N) but this has not been thoroughly studied in agricultural plants. A critical question is whether agricultural plants can acquire amino acids in a soil ecosystem. The aim of this study was to characterize amino acid uptake capacity in barley (Hordeum vulgare L.) from a mixture of amino acids at concentrations relevant to field conditions. Amino acids in soil solution under barley were collected in microlysimeters. The recorded amino acid composition, 0–8.2 μM of l-Serine, l-Glutamic acid, Glycine, l-Arginine and l-Alanine, was then used as a template for uptake studies in hydroponically grown barley plants. Amino acid uptake during 2 h was studied at initial concentrations of 2–25 μM amino acids and recorded as amino acid disappearance from the incubation solution, analysed with HPLC. The uptake was verified in control experiments using several other techniques. Uptake of all five amino acids occurred at 2 μM and below. The concentration dependency of the uptake rate could be described by Michaelis–Menten kinetics. The affinity constant (K
 m) was in the range 19.6–33.2 μM. These K
 m values are comparable to reported values for soil micro-organisms.

Characteristics of amino acid uptake in barley

In this study, we present a rapid, robust and sensitive method for quantification of plant amino acid uptake using universally (U) (13C, 15N)-labelled amino acids and gas chromatography-mass spectrometry (GC-MS). Amino acids were analysed as their tert-butyldimethylsilyl (tBDMS) derivatives and displayed detection limits in the range 10-100 fmol on column, depending on the amino acid. The technique allows for simultaneous detection and quantification of both unlabelled and isotopically labelled species of amino acids. This makes simple quantification of plant amino acid uptake from an isotopically labelled source possible. The analytical variation was low, concerning total amino acid concentrations (relative standard deviation, rsd, less than 5.3%) as well as enrichment of U-13C, 15N-labelled glycine (Gly), arginine (Arg) and glutamic acid (Glu) (rsd<2.1%). An application of the GC-MS method was conducted on non-mycorrhizal Pinus sylvestris roots supplied with U-13C, 15N-labelled amino acids. Intact, labelled amino acids were traced in root extracts. This provided conclusive evidence of plant root uptake of intact amino acids. Uptake rates of the three amino acids Gly, Glu and Arg in the range 0.5-37.9 mmol g-1 dry weight h-1 were recorded. These rates are comparable with those recorded in earlier studies of amino acid uptake, using other methods, as well as uptake rates measured for nitrate and ammonium.

A GC-MS method for determination of amino acid uptake by plants.

This study was conducted to answer the question of whether clover can absorb asparagine in the presence and absence of inorganic nitrogen, as well as to determine the resulting concentration of post-uptake compounds closely involved in asparagine metabolism. Clover was grown at two asparagine concentrations (10 μM and 1 mM) supplied in both the absence and presence of ammonium nitrate. Using dual-labeled 13C15N-asparagine, the uptake rate was analyzed via bulk 15N and 13C excess and the detection of intact 13C15N-asparagine in white clover. The results from the two methods indicated greater utilization of 13C15N-asparagine in the 10 μM treatment than in the 1 mM treatment. The 13C15N-asparagine uptake rate was higher when 13C15N-asparagine was provided alone than when it was supplemented with inorganic nitrogen. Up to nine times lower uptake rates were obtained when intact 13C15N-asparagine was measured than when bulk 15N and 13C excess were analyzed. The labeled amino acids that are closely related to 13C15N-asparagine metabolism (aspartic acid, glutamic acid and glutamine) were detected in clover roots and shoots. Using two different methods, white clover’s potential to absorb intact asparagine, even in the presence of inorganic nitrogen, was confirmed. The dual-methodology approach employed in this study demonstrates how the post-uptake metabolism can affect quantification of amino acid uptake.

/pdf/direct-acquisition-of-organic-n-by-white-clover-even-in-the-3m78wqd8ko.pdf

Torgny Näsholm

Papers

Uptake of organic nitrogen by plants.

Growth of conifer seedlings on organic and inorganic nitrogen sources

Characteristics of amino acid uptake in barley

A GC-MS method for determination of amino acid uptake by plants.

Direct acquisition of organic N by white clover even in the presence of inorganic N